The following background information is presented only by way of example with reference to thermal transfer printing. Thermal transfer printing entails the controlled transfer of an ink (e.g., a colorant dispersed in a wax base material) from a carrier such as a polymer ribbon onto a print medium surface. A thermal transfer printer having a print head with a large number of independently activable heating elements per unit of length is one prior art apparatus employed for this purpose. The ink/carrier structure is placed within the printer such that the carrier side is adjacent the heating elements and the ink side is adjacent a print media support upon which the print medium rests during printing.
To print an image, the print head contacts the polymer ribbon and ink is transferred to particular locations on the print medium surface when predetermined combinations of heating elements are activated adjacent the image-forming locations. The ink/carrier structure is locally heated by the heating elements to a temperature at or above the melting point of the ink. In this manner, an amount of ink softens and adheres to the print medium at the predetermined locations to form the image.
Color images are printed with an ink/carrier structure that includes separate regions of differently colored inks such as the subtractive primary colors, yellow, magenta, and cyan. Color printing is accomplished by sequential passes of the print medium past the print head, each pass selectively transferring different colored inks at predetermined times. Thermal printing ribbons are available with a single black panel, three color panels (yellow, magenta, and cyan), or four color panels, (yellow, magenta, cyan, and black).
Many printers include a control software driver program (hereafter "printer driver") for handling various aspects of the printer operation. Such printer drivers are often interfaced to a computer programming language known as PostScript.RTM., which is available from Adobe Systems Inc., Mountain View, Calif. The PostScript.RTM. language, described in the PostScript.RTM. Language Reference Manual, Second Edition, 1990, Addison-Wesley Publishing Co., Reading, Mass., includes methods for manipulating text and graphics, selecting media sizes, types, trays, and the number of copies to be printed.
A thermal transfer printer typically transfers quantities of ink of a single volume that produce on a print medium dots of ink sized to provide "solid fill" printing at a given resolution, such as 300 dots per inch ("dpi") 12 dots per millimeter ("dpm"). Single dot size printing is acceptable for most test and graphics printing applications not requiring "photographic"image quality. Photographic image quality normally requires a combination of high dot-resolution and an ability to modulate a reflectance (i.e., gray scale) of dots forming the image.
In single dot size printing, average reflectance of a region of an image is typically modulated by a process referred to as "dithering" in which the perceived intensity of an array of dots is modulated by selectively printing the array at a predetermined dot density. For example, if a 50 percent local average reflectance is desired, half of the dots in the array are printed.
An image is formed by the tessellation of multiple cells or "tiles," which are clusters of pixels arranged in a predetermined pattern. The pixels correspond to dots printed on a page. Each cell has the same number of pixel locations arranged in the same pattern. The number of pixels filled in a cell determines its darkness, and there is a specified "fill order" for darkening certain pixels in a cell as its darkness increases. The number of pixels selected for a cell presents a tradeoff between the resolution and number of available gray levels of the resulting image. A greater number of pixels reduces image resolution but increases the number of available gray levels. Replication of tessellated cells of varying darknesses form the printed image.
An important characteristic of thermal wax printers is their inability to place individual dots independently of neighboring dots. This characteristic, which is shared by other common printing technologies, is complicated by a high sensitivity to the thermal history of the print head.
The lack of pixel independence imposes an ordering of clusters. Clustered-dot halftoning partitions the addressable area into a relatively low frequency tiling of the printed page, as compared with the resolution of the device. Each one of multiple pixel clusters is populated from the center out to simulate an oval spot of variable size of the sort printed (but at a much higher frequency than dots printed) on a printing press.
The tendency to simulate conventional printing methods is reinforced in PostScript.RTM. in which the primary, and originally the only, way to specify a digital halftone was with a screen angle, frequency, and spot fill function (normally a simple distance from center function) for each primary color. Working within the framework of the Postscript.RTM. halftone screen specification, several significant improvements have been implemented to improve the quality of thermal wax output on Postscript.RTM. compatible Tektronix printers. These improvements include a spiral spot (dot cluster) growth pattern to reduce unwanted cluster-to-cluster interactions; aligned halftone screens to avoid moire (rosette) patterns; and "super-cell" grouping of spots to increase tint levels while retaining high frequency detail.
FIG. 1 shows a prior art cell grouping of twenty-nine pixels arranged in a square pattern with pixel attached to each vertex of the square as shown. The numbers identifying each pixel in FIG. 1 represent the order in which the pixels in the cell are darkened and indicate a spiral spot growth pattern starting from pixel 0 in the center and moving counter-clockwise around pixel 0 to pixels 25, 26, 27, and 28 at the vertices. FIG. 2 shows a prior art super-cell structure and spot growth visitation order for an array of nine halftone cells of the type shown in FIG. 1. FIG. 3 shows three super-cell structures of FIG. 2 arranged to define a prior art super-cell tessellation geometry. FIG. 4 shows an image halftoned using the screen corresponding to the tessellation geometry of FIG. 3.
The halftoning method depicted in FIGS. 1-4 was implemented on, for example, the Phaser PX, Phaser PXi, Phaser II PXi, and Phaser 200 printers manufactured by Tektronix, Inc., the assignee of this application. These printers had 300.times.300 dpi (12.times.12 dpm) addressability and exhibited good image quality because of the super-cell spiral spots.
The Phaser 200, a faster and less expensive thermal wax printer, presented certain new problems when used with the prior halftoning technique described above. The faster print speed set new limits on dot geometries that would produce consistent results. The relatively complex cell shapes used in the prior technique caused nonuniform darkness changes across tint levels. These were compensated for with gamma correction to produce an acceptable result, but at the cost of a significant reduction in the number of tint or gray scale levels.
In a subsequent Tektronix product, the Phaser 220, the addressability was increased to 600 dpi (24 dpm) in the direction of paper motion to increase printer resolution, but dot size remained the same. To take full advantage of increased resolution, a much higher frequency halftone pattern was needed. In addition, a new pattern would be needed to accommodate overlapping dots, an asymmetric grid, and the geometry limitations described above. Such needs cannot be met by the existing halftone method.